Art. VII.—On the Reclamation of Waste River Beds.
[Read before the Philosophical Institute of Canterbury, 2nd June, 1881.]
One of the most striking features of the Canterbury Plains is the great area of ground occupied by the shingle beds of the rivers, an area quite out of proportion to the size of the rivers were they confined so as to flow in one deep channel instead of spreading as they now do in numerous shallow streams over their wide shingle beds.
From the Waipara to the Rangitata the area of the shingle beds is at least 100,000 acres, one half of which would be amply sufficient for the water-way were the rivers properly regulated. Not only do the rivers occupy a much greater area than is at all necessary for the water-way, but in the lower part of their courses they are continually filling up their beds with the shingle brought down from the mountains, and cutting fresh channels in the adjoining lands, thus destroying valuable property, and being a continual source of expense and anxiety to those who live on their banks.
The Waimakariri in Canterbury, and the Wairau in Marlborough, are well-known illustrations of this statement.
All rivers have a tendency to raise their beds in the lower part of their courses, this action being slowest when fine silt only is brought down, such as that deposited by the Nile and Mississippi, and most rapid in shingle-bearing torrents, such as our own rivers.
Even in the case of large rivers with long courses and little fall, the rise of the river bed, and often the adjoining overflowed country, is more rapid than would be expected by a casual observer. From long before the historical period, the Nile has been steadily raising its bed, but as its
flood-waters were the source of the fertility of lower Egypt, river works were undertaken with a view to obtain the advantage of the overflow on the greatest possible area, and the flooded lands were raised annually by this process, as well as the river bed itself.
This cannot happen in the case of torrents carrying large quantities of shingle and sand, which are brought down in times of flood and left in the channels as the velocity of the stream decreases, and although much may be done towards raising the adjoining lands by inducing an overflow of flood water upon them, the floods will always bring down from the upper levels masses of material that it has no power to move along the lower course.
This is the problem the Italian Engineers have had to deal with in the management of the Po and its tributaries.
The general system in Europe for conservation seems to be to wall in the river between embankments with the endeavour to make it carry its silt as far forward as possible, making traps for catching shingle; where possible raising adjoining lands by ponding up flood-water upon them; and many other expedients for keeping the rivers from changing their courses and wandering about their fans into new courses. The great drawback to this treatment is, that however much (within certain limits) the river may be embanked, it continually raises its bed, and the banks must be raised in proportion.
To such an extent has this been done that some of the Italian rivers are now far above the level of the adjoining lands, so that the surface drainage of the country has to be carried in other channels, involving all kinds of complications in the drainage; and should the embankments fail, immense volumes of water may be dammed up on the low-lying land by the banks which were built for their protection, entailing great loss of life and property. Such a disaster occurred not long since in Hungary, at Szegedin, on the River Theis.
When we consider the enormous cost that would be incurred in attempting to control our great torrents by any system of solid embankments, it becomes apparent that we must either invent some plan which can be carried out on a much cheaper scale, or else let matters take their course.
Doubtless weak places may be defended by groins and spurs; and the more chance will there be of success so long as only one side of a river is being worked upon. But so soon as works are undertaken on both sides of a river, they will have to be more or less continuous embankments, which must be made of permanent materials, involving immense expense.
On the Canterbury Plains the conditions are most favourable for the formation of broad shingle beds, the loose shingle banks offering no resistance
to the action of the floods. In a bush country, in its natural state, quite other conditions obtain; the vegetation along the river, so long as it is uninjured by stock or fires, affords great protection to the banks. Flood-water overflowing the banks is checked, and the silt deposited, whilst all scouring action is stopped by the roots and vegetation covering the surface of the ground; and when in the course of time the banks wash away, the vegetation at once takes possession of all the shingle-spits, so that in the end the forest recovers the ground lost, and the mean area of shingle remains the same; and also the silt brought down during floods settling amongst the shrubs and trees on the banks, raises them as well as the river bed. This is the way in which nature utilizes the shingle beds in bush countries; and it appears to me that by following the same mode a vast area of profitless shingle bed could be turned to account, and the rivers kept within bounds at the same time.
I consider planting would be within the means of the country; and, if properly carried out, would eventually turn valueless shingle beds into valuable forest.
I would plant a belt of willows along the river in the first instance, and, as these grew, the planting should be carried out on all the higher spits and islands. The willow-planting would be done very cheaply, a stout willow-stick being put down in a hole made with an iron bar. As soon as the willows grew, they would rapidly collect silt, and, on the ground thus formed, trees of useful varieties suited to the locality should be planted. Many flats which are only flooded slightly, on very rare occasions, could be planted at once; and in a few years the present shingle deserts would be turned into a wide expanse of young forests, with the river meandering through them. In addition to the planting, it would be necessary to erect protective works in some cases, where there was danger of the river destroying the plantations before they were strong enough to protect themselves.
In a few years the willows would provide ample material on the spot for the construction of brushwood groins, and a vast amount of live protective works could be constructed at a small cost.
It is to be expected that the plantations would suffer from time to time by the floods; but, as the banks got well covered with vegetation, the destruction of the river banks would be very slow, and the trees washed away would lodge on shingle banks and commence recovering land without further help.
By utilizing the shingle beds in the manner just described, I consider two advantages would be gained,—the formation of a useful forest, and the regulation of the river so planted,—as the forest itself would form one of
the best protective works that could be made. By no other system could protective works be made a source of profit, and the money expended must be considered as so much capital sunk, the yearly interest on which represents the price the country pays for the privilege of occupying the endangered land.
The next question is that of cost, which can only be arrived at approximately, as no planting has ever been undertaken in New Zealand on such a scale as that now proposed under similar conditions; but, probably, £15 per acre would be sufficient, including the cost of fencing. At the end of ten years the thinning-out should be worth at least £1 per acre (for fence stakes and firing), clear of expenses for management and renewals; and at the end of twenty years, the timber would be large enough for posts and rails, scaffold poles, and mining timber.
Assuming that 40,000 acres could be easily utilized, the cost would be approximately, as follows:—40,000 acres, at £15 per acre, £600,000; to this add, say, £100,000, for groins, wing-dams, etc.: making a total of £700,000.
For ten years the interest on this sum will be lost; but, at the end of that time, the returns should be at the rate of £1 per acre per annum, which equals £40,000, or a trifle over 5½ per cent, on the £700,000, and the profits would increase as the trees grew larger.
The presence of the forests would, doubtless, exercise a very beneficial influence upon the climate of the plains, checking the north-west winds and inducing a greater rainfall.
There are many difficulties at present in the way of carrying out such a scheme as the foregoing, but not such as would offer any insurmountable obstacle, if the matter was taken seriously in hand.
It would be necessary to have one Board of Conservators for each river, possessing full power to fence, take land, and carry out everything necessary for the work.
There has been a great deal of planting done in Europe during the last fifty years,—the greatest achievement being the planting of 150,000 acres in the south-west of France. The whole of this area was planted in twenty-five years, at the end of which time a quantity of mining timber was being exported to England, and factories were being erected for the manufacture of paper and distillation of turpentine.
By following such an example as this, we may reclaim our barren shingle beds, control the rivers through a considerable portion of their course, and lay the foundations of extensive and profitable local industries.